A light emitting diode (LED) is a solid state device that converts electrical energy to light. Light is emitted from active layers of semiconductor material sandwiched between oppositely doped layers when a voltage is applied across the doped layers. In order to use an LED chip, the chip is typically enclosed in a package that focuses the light and that protects the chip from being damaged. When LEDs are packages in arrays as opposed to as discrete light emitters, the LED chips of the arrays are mounted directed on a printed circuit board without the carrier substrate conventionally used with discrete light emitters. The LED chips packaged as arrays are electrically connected to contact pads and to traces in a top trace layer of the printed circuit board. The LED chips are wire bonded to the traces on the top side of the printed circuit board. The printed circuit board is then segmented to form discrete array light sources. Larger exposed areas of the traces on the top side form contact pads to which supply power is connected to each discrete array light source.
The LEDs are typically covered with a layer of phosphor before the array light sources on the printed circuit board are segmented. The phosphor converts a portion of the blue light generated by the LEDs to light in the yellow region of the optical spectrum. The combination of the blue and yellow light is perceived as “white” light by a human observer. Before the array light sources are segmented, the LEDs are typically covered by a layer of silicone that is formed into a lens above each light source. The layer of silicone also protects the LED chips and top-side wire bonds.
A slurry containing the phosphor has been conventionally dispensed manually into a ring or dam around the LED chips of each array light source. Then injection molding or casting molding has been used to form a lens above each array light source. The manufacturing process for LED light sources has been improved by combining the steps of dispensing the phosphor and forming the lens. By adding the phosphor to the silicone, the separate step of dispensing phosphor can be eliminated, and lenses are formed with phosphor dispersed throughout each lens. The lenses are formed using injection molding in which lens cavities that contain the LED dies are filled with the lens material, and the excess lens material is squeezed out of a leakage path.
When casting molding is used, a phosphor silicone slurry is first dispensed into the bottom half of each cavity, and then the top half of the cavity closes to define the lens structure and squeezes out the excessive lens material. The injection molding and casting molding processes have multiple disadvantages. First, the phosphor and the silicone are expensive, and the lens material that is squeezed out of the cavities is wasted. Second, the quality of the lenses formed with injection molding and casting molding is low because bubbles and nonuniformities remain in the finished product.
These disadvantages can be overcome by using compression molding in which the lens material is contained in a single sealed cavity under compression. Placing the lens material initially under a vacuum and then under high pressure between the two parts of the compression mold ensures a uniform consistency of the lens material throughout the cavity and prevents bubbles from forming. Moreover, only the amount of lens material that is actually used is pumped into the sealed cavity, so no lens material is wasted.
Unfortunately, at least one complication must be overcome before compression molding can be used to form lenses over LED array light sources on printed circuit boards. The compression molding process relies on the lens material being able to flow freely through a flash layer between the individual lens cavities so that the lens material is uniformly distributed. Consequently, the entire surface of the printed circuit board between the lenses is covered by the flash layer. So the contact pads for each LED array on the top side of the printed circuit board are covered by the flash layer of silicone, which inhibits an electrical contact being made with the contact pads. Existing compression molding techniques require the flash layer to be at least fifty microns thick, whereas the trace layer that forms the contact pads can be as thin as a couple of microns. Manually scraping off the flash layer would either damage the contact pads or not remove the silicone from the entire surface of the contact pads. Taping over the contact pads before the compression molding step and then later removing the tape would increase the cost by adding two additional steps. In addition, the silicone at the edges of the lenses could tear when the tape is lifted.
An efficient method is sought for removing the flash layer of silicone that results from compression molding without damaging either the lenses or the trace metal layer that forms the contact pads on the top side of the printed circuit board.